Many different designs of surface recession sensors have been fabricated and tested in entry vehicle applications for the Department of Defense, ranging from simple thermocouples and “breakwire” arrangements to more complicated devices such as radiation sensors. In a breakwire sensor arrangement, multiple thermocouples or pairs of wires are embedded in a thermal protection system (TPS) plug with their junctions located at different depths that will sense different local temperatures, as shown in FIG. 1.
Theoretically, as the TPS surface recedes and/or chars, the increasing heat progressively melts each wire and breaks the circuit. However, in practice it is found that the receding char layer becomes conductive and serves as an electrical contact between the melted wires which leads to erroneous conclusions that the circuit has not yet broken. In radiation transducer recession sensors, radioactive particles are embedded in a TPS material at selected depths. The strength of the radiation measured by a Geiger counter decreases as more radioactive particles are removed due to surface recession.
One of the more successful efforts in the development of a surface recession sensor is the quartz-based Analog Resistance Ablation Detector (ARAD), originally developed by General Electric. ARAD had been tested in three-dimensional quartz phenolic. The NASA probe heat shield was instrumented with ARADs to monitor the probe's heat shield shape and mass loss during its entry into Jupiter's atmosphere. The ARAD design implemented for used the same operating principal as the quartz-based ARAD but with different materials. Though there were some issues in the measured data, a NASA version of ARAD met scientific objectives because it provided the valuable information that the ablation was substantially axisymmetric, and the returned data allowed the final heat shield shape to be reconstructed.
Several improvements in accuracy and flexibility of the system are needed. One or more methods for checking consistency of the recession measurements is needed, and an increase in resolution of these measurement is needed to identify anomalies that may occur in the measurement process.